It is easy to calculate the
overall concentration of steel fibres in a concrete batch seeing as the mass of
fibres used and the volume of the batch are both known. However, it is more
difficult to discern how well the fibres are distributed throughout the batch,
which is important to know as poor distribution could lead to weak points in
the hardened concrete. The wash out test is a quick way to test multiple parts
of the batch to see if the concentration of fibres is consistent throughout,
though it must be noted that test results are not conclusive and should be
regarded as indicative of fibre distribution only.
The method is as follows:
- A known volume of concrete from the batch is placed in a
box fitted with three sieves.
- The concrete is wet sieved so that most of the concrete
itself is washed away.
- The fibres are collected from the sieves by a magnet.
- The fibres are washed and weighed.
- At least two more samples are taken from different parts
of the batch and tested in the same way.
The national standards for this test are BS EN 1170 and ASTM C1228 in
the UK and USA respectively.